Sports Cycle

Valve Clatter : The Top End

The top end of a motorcycle is where the internal-combustion process takes place. The top end consists of the pistons, the cylinder block, and the head, which contains the valve train. As mentioned earlier, pistons are metal slugs that move up and down in the cylinders. The energy created by burning the fuel charge (a mixture of gasoline and air) acts on the tops of the pistons (known as the crowns) to make your bike move. Pistons are both simple and complex devices, but for the sake of this discussion, I’ll stick with this simple definition.

The cylinder block is a rather simple device, too—a hunk of aluminum with holes bored through it, inside which the pistons move up and down. This hunk serves two purposes :

1. It channels the pistons’ energy and forces them to move up and down (rather than flying all over the place, which would not help make your motorcycle move).
2. It dissipates the intense heat created by the burning fuel charge and the friction caused by the motion of the pistons. This is done by some sort of coolant circulating throughout the block, by cooling oil being sprayed on the hot spots on the piston, or by air flowing over the outside of the block. Usually, some combination of the three methods effectively cools the engine.

The valve train is the most convoluted and confusing system in the top end. It consists of a cam, which is a rod with lobes on it that opens the valves. Valves are devices consisting of metal stems with flat disks on one end. They ride in metal tubes, called valve guides, and open and close at precise moments to let unburned fuel charges into the combustion chambers and to let the waste gases created during the combustion process exit the combustion chambers. There are two basic types of valve trains: overhead cam systems and pushrod systems. There are several varieties of each system, but they all share some common traits.

Most modern bikes use overhead cams. In this type of valve train, the cams are located above the valves and act on them through a system of rocker arms, or the cam lobe acts directly on the valve. Rocker arms are metal levers that rotate on a small shaft like an upside-down teeter-totter; the cam lobes push one side of the rocker up, causing the other side of the rocker to push the valves down, opening them up.

The pushrod system has cams located below the combustion chambers, usually in the engine cases, that act on pushrods, long, metal rods connected to the rocker arms located above the valves. Harley-Davidson and Moto Guzzi are the only two major manufacturers still using this method of valve actuation in its traditional form. BMW uses a hybrid camin- head system, which locates the cam in the cylinder head but below the valves. This system uses short pushrods to open the valves.

Overhead cams have a more positive action (less slop in the system) and allow the engine to operate at a higher rate of revolutions per minute (RPM) than can pushrod engines. The term RPM refers to the number of times a crankshaft spins around each minute. Being able to run at higher RPMs means an engine will put out more overall power—the faster your crankshaft can spin, the faster your rear wheel will turn, and the faster you will be able to go.

One advantage of Harley’s pushrod system is easy maintenance. Harleys use hydraulically adjusted tappets (small metal slugs between the cams and the pushrods) that eliminate the need for periodic valve adjustments. Some overhead-cam designs also use hydraulic-valve adjustments, but most require periodic manual adjustments.